Field of the Invention
The present invention relates to the field of glass-ceramics. More specifically, it relates to a glass-ceramic article (or product), notably a glass-ceramic plate, intended, in particular, to cover or accommodate heating elements, said article being provided with a colored luminous display (or at least one colored luminous/illuminated area) in at least one chosen area of the article.
Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
The sales of articles such as glass-ceramic cooktops have been steadily increasing for a number of years. This success is due, notably, to the attractive appearance of these plates and their ease of cleaning.
Let us remind that the starting material for a glass-ceramic is a glass, designated by precursor glass (or mother glass, or green glass), the specific chemical composition of which allows to provoke controlled crystallization by suitable heat treatments known as ceramization. This special partly crystallized structure imparts unique properties to the glass-ceramic.
There exists at present different types of glass-ceramic plates, each variant being the result of considerable research and numerous experiments, since it is extremely difficult to modify these plates and/or their production process without risking an unfavorable effect on the looked for properties: in order to be used as a cooktop, a glass-ceramic plate must generally present a transmission in the visible range wavelengths which is low enough to mask at least part of the underlying heating elements when they are inactive, while also being sufficiently high so that, depending on the situations (radiant heating, induction heating, etc.), the user can visually detect the heating elements when active in the interests of safety; at the same time, it must provide high transmission in the infrared range wavelengths, notably in the case of plates with radiant heating elements.
The most common plates at the present time are dark in color, particularly black, and are colored, for example, by using vanadium oxide added to the raw materials of the mother glass before melting, this oxide imparting a sustained orange-brown tint after ceramization, due to reduction of the vanadium. Other coloring agents, such as oxides of cobalt and manganese, can also be used. With a low transmission coefficient of less than 600 nm, these plates mostly allows to view red elements such as heating elements raised to a high temperature or luminous displays based on red monochromatic light-emitting diodes. There are also more transparent glass-ceramic plates (such as the KeraVision or KeraResin glass-ceramic marketed by the EuroKera company) existing, and which allow the display of other “pure” colors (produced by monochromatic diodes), such as blue or green.
There has nevertheless appeared recently the need to be able to display a greater variety of displays with more varied colors, and, in particular, with synthetic colors produced by mixtures of a plurality of wavelengths (as in the case of the white color). Since the transmission coefficient of glass-ceramic plates is not uniform over the whole visible spectrum, the relative amplitudes of the different (spectral) components of the transmitted light are yet generally modified, and the color after transmission may differ greatly from that produced by the source.
In particular, technologies based on light-emitting diodes (LEDs) which are commonly used to produce white light (for example, with a blue light source covered with an element absorbing part of said light and re-emitting yellow light) cannot be used to produce a white color through a glass-ceramic. While the balance between the blue and the yellow is initially such that their mixture produces a visual sensation of a white color, then, because of the passage through the glass-ceramic, the absorption not being uniform (blue is strongly absorbed, and yellow less absorbed), the eye does not perceive anymore white through the glass-ceramic, but perceives, for example, pink, orange or red.
Similarly, the use of LEDs with polychromatic emission (for example, those formed by three monochromatic sources having independently adjusted intensities, such as LEDs of the “RGB” type with three sources: red, green and blue) to provide, for example, white, is not appropriate, the non-uniform absorption of the glass-ceramic in the visible range disrupting the balance between the colors and producing as well a pink, orange or red appearance. The respective intensities of the RGB components can be adjusted, but the mixture must be perfect (notably in spatial terms—good coverage of the light beams—and in temporal terms—same phase of a possible amplitude modulation of the beams notably,) to avoid non-uniformities; the spacing of the three emission areas often leads to poor mixing, resulting in a non-uniform color. Similarly, the three R, G and B chips are subject to different thermal drifts and aging, allowing colorimetric non-uniformity to develop over time. Furthermore, color variations between one RGB LED and the next are also observed, depending on the manufacturing batches of the red, green and blue LEDs. RGB LEDs are also more bulky than the LEDs normally used for display units, and are harder to integrate into a control panel.
For these reasons, displays in white or in most colors other than red, particularly in synthetic colors, are not found in glass-ceramics, notably with dark or colored glass-ceramics, because of their non-uniform absorption in the visible range, a non-monochromatic light passing through them having its color modified, this being all the more critical as its spectrum is wide, as in the case of white sources.